Electric power distribution networks are used by the electric utilities to deliver electricity from generating plants to customers. The actual distribution voltages will vary from country to country and from utility to utility within a given country. In a typical power distribution network in the U.S., three-phase power at a high voltage (e.g., 345 kilovolts phase-to-phase) is delivered to multiple transmission substations. At these transmission substations, this high-voltage power is stepped down to an intermediate three-phase voltage (e.g., 69 kilovolts phase-to-phase). The intermediate-voltage three-phase power from each transmission substation is then delivered to multiple distribution substations. At the distribution substations, the intermediate-voltage is stepped down to a lower distribution voltage (e.g., 12.47 kilovolts phase-to-phase) and separated into three single-phase feeder lines (e.g., 7.2 kilovolts phase-to-ground). Each of these feeder lines branches into multiple circuits to power a plurality of local pole-mounted or pad-mounted transformers that step the voltage down to a final single-phase voltage of 120 and 240 volts for delivery to the commercial and residential customers.
Ideally, the utilities try to initially design the feeder circuits such that the loads are balanced, i.e., the current loads on each single-phase output of a three-phase transformer are equal. However, over time, as customers are added and removed, the loads on each single-phase output may change and become unbalanced. To re-balance the loads, some of the branch circuits are typically moved from a more heavily loaded phase to a more lightly loaded phase.
To re-balance the loading, the phase of each line in a distribution cabinet must be accurately known. Otherwise, a line may be erroneously removed from a more lightly loaded phase and placed on a more heavily loaded phase. If this happens, the procedure will have to be repeated, which will cause a second disruption in service to all customers on the branch being re-phased. In the worst case, adding a greater load to the more heavily loaded phase may cause a substation fuse to blow, resulting in a power outage for all customers on the overloaded phase.
Currently, to accurately identify the phase of a particular feeder branch, utility company personnel must physically trace a line run back through various distribution cabinets until they reach a point in the distribution network at which the phase is definitely known. This can be a time consuming, labor-intensive process.
Various devices and methods have been described to assist in the phase identification of lines. Bouvrette, U.S. Pat. No. 4,626,622, proposes using modems and telephone lines to establish a communication link. A signal associated with the phase at a point in the network where the phase of the line is known (the reference line) is transmitted over the communication link to a point in the network where the phase of the line is not known (the line under test). Difficulties arise with this methodology in that delays in the communication link may severely affect the accuracy of the phase measurement.
Pomatto, U.S. Pat. No. 5,510,700, proposes a similar scheme to that of Bouvrette save that the communication link uses radio transmissions to eliminate communication-link delay problems. However, both Bouvrette and Pomatto require calibration procedures and special training for their techniques to be used effectively.
Hao, U.S. Pat. No. 6,130,531, proposes a method and apparatus to compare phases between electric power system substations in real time. His method is similar to that of Bouvrette and Pomatto except that Hao uses a time base locked to Global Positioning System (GPS) time at both the reference line and the line under test to eliminate delay and synchronization problems.
Finally, a phase identification instrument is currently available that, like Hao, uses GPS receivers at both the reference line and the line under test to eliminate delay and synchronization problems, and, like Bouvrette, uses a cellular data communication link to convey the phase of the reference line to the line under test location. This instrument transmits the instantaneous phase of the reference line once each GPS second. The instantaneous phase of the line under test is measured at a given second, then compared to the phase of the reference line for that same second. All communication and readings are performed in the same one-second interval. This necessitates communication delay is less than one second.
In all these approaches, a pre-established real-time communication link is required. That is, a communication link needs be established and active at the time the phase of the line under test is measured. This renders these approaches unusable wherever and whenever the communication link cannot be established. Also, since the phase of the line under test is determined for each measurement, the measuring apparatus must be retrieved after each test. This precludes the ability to make several different tests before accessing the apparatus, e.g., measuring the phases of several different overhead lines in a substation before lowering the “hot stick” to which the apparatus is attached.
Accordingly, it is the object of the present invention to provide a new and improved apparatus and method for the identification of line phase of a power line in a three-phase power distribution network. The apparatus and method do not require a pre-established real-time communication link, do not suffer from communication delay and synchronization difficulties, do not require calibration procedures to be performed, and do not require special training on the part of the operator.